The presence and role of transforming growth factors (TGFs) in human mammary carcinoma cells is being defined. Alpha-TGFs resemble epidermal growth factor (EGF) in that they are able to compete with EGF mouse and human EGF but fail to cross-react with antibodies generated against these two species of EGF. Alpha-TGFs can be recovered from the acid-treated, concentrated conditioned medium (CM) of a human mammary carcinoma cell line, MCF-7, and ten individual clones derived from this cell line. The level of TGF activity associated with the CM varied amongst the different MCF-7 clones and showed no correlation with either the number of EGF receptors expressed on these cells or with the intrinsic ability of these clones to grow in soft agar as colonies. The TGF in the CM is an acid heat stable peptide whose activity is destroyed by reduction. It has a molecular weight of approximately 6000 and a pI of 4.0 TGF activity can also be detected in the crude, acid-ethanol extracts prepared from MCF-7 cells propagated as tumors in nude mice, in the acid-ethanol extracts obtained from two transplantable human mammary adenocarcinomas, Clouser I and II, and in 3 out of 4 primary human breast carcinoma samples. Relatively high levels of TGF activity can be found in crude, delipidated, decaseinated human milk obtained from at least 30 individual donors. The levels of TGF in the milk samples vary from individual donors and are generally highest in clostrum. Following isoelectric focusing (IEF), three distinct TGF species can be detected in the human breast tumor samples with identical species found in human milk. The TGF with a pI of 3.8 - 4.0, mammary-derived growth factor-II (MDGF-II), has been purified approximately 10,000-fold. This alpha-TGF species is virtually identical to the species found in the CM from MCF-7 cells. This acidic alpha-TGF is biologically and physiochemically distinct from the major species of human EGF in milk.